Radio frequency (RF) heating of fusion plasmas in the ion cyclotron range of frequencies (ICRF), typically between about 10 and 200 MHz, is now being widely applied to fusion experiments around the world. It is currently invisioned that fusion reactors will use this method of heating to supplement ohmic heating and neutral beam heating. Power levels are now in the multimegawatt range and experiments are frequently being limited by breakdown at the vacuum feedthrough, i.e., the RF coupling between a pressurized coaxial transmission line and the plasma vacuum containment vessel. This barrier between the pressurized line and the evacuated line within the vacuum vessel is a particularly crucial component because its failure affects not only the RF system but also the entire vacuum integrity in many circumstances. This component has also been the weak link in voltage handling for some contemporary pulsed experiments. The potential problems at the feedthrough are compounded by operation approaching steady-state.
Various development programs have been undertaken to develop feedthrough designs for specific applications. One such program has been underway at the Princeton Plasma Physics Laboratory for a number of years. Their efforts have led to the development of a high-power feedthrough used in ICRF heating experiments on the Princeton Large Torus (PLT). The PLT feedthrough is the subject of the U.S. Pat. No. 4,484,019 issued Nov. 20, 1984, to Glenn F. Grotz for a "High Voltage RF Feedthrough Bushing" and having a common assignee with the present invention, the contents of which are incorporated herein by reference thereto. The PLT feedthrough uses a conical ceramic barrier between inner and outer coaxial conductors. The conductors are shaped primarily to reduce the component of the electric field along the surface of the ceramic. The conical-shaped barrier is difficult to manufacture and assemble into a connector to obtain matched impedance and eliminate internal reflections. Further, the PLT feedthrough is limited in operation to RF frequencies below about 200 megahertz.
Thus, it will be seen that there is a need for an improved RF vacuum feedthrough for use at higher frequencies and power levels which is less complicated in design, provides matched impedance along the length of the feedthrough and minimizes internal reflections.